In the prior art literature there have been described processes for modifying with derivatives of silanic type the surface of filler or pigment materials, such as SiO.sub.2, TiO.sub.2, aluminas etc. employed in many thermoplastic and thermosetting materials, to improve the adhesion between the fillers or pigments and the other components. The filler or pigment so obtained may be finally coloured with conventional dyes. Such conventional dyes include common reactive or acid polysulphonic dyes, which do not contain silane residues and are soluble in water. The dyes of the present invention are s-triazinyl-bis-anthraquinones and are different from the aforementioned previously known dyes. Furthermore, the method for preparing the earlier mentioned coloured prior art pigments is substantially different from the one used for preparing the pigments according to the present invention.